Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha Reservation schemes Collision avoidance, MACA Polling CDMA SAMA Comparison Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.1

Motivation for a specialized MAC Can we apply media access methods from fixed networks? Example CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, by listening into the medium if a collision occurs while sending ,it stops at once and sends a jamming signal It is not a problem in using a wire, as more or less the same signal strength can be assumed all over the wire if the length of the wire stays within certain often standardized limits (original method in IEEE 802.3) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.2

Problems in wireless networks signal strength decreases proportional to the square of the distance to the sender..(obstacles attenuate the signal even further) the sender would apply CS and detect an idle medium., sender starts sending – but a collision happens at the receiver due to a second sender sender detects no collision and assumes that the data has been transmitted without errors, but a collision might actually have destroyed the data at the receiver. Collision detection is very difficult in wireless scenarios as the transmission power in the area of the transmitting antenna is several magnitudes/degrees higher than the receiving power (receiver side). it might be the case that a sender cannot “hear” the collision, i.e., CD does not work this very common MAC scheme from wired network fails in a wireless scenario. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.3

Motivation for a specialized MAC Hidden terminals The transmission range of C reaches B, but not A. Finally, the transmission range of B reaches A and C A sends to B, C cannot receive A (the detection range does not reach C either) C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.4

Motivation for a specialized MAC Near and far terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns(go down) out A’s signal C cannot receive A If C for example was an arbiter(C acts as a base station coordinating media access) for sending rights, terminal B would drown out terminal A on the physical layer The near/far effect is a severe problem of wireless networks using CDM. All signals should arrive at the receiver with more or less the same strength. Otherwise a person standing closer to somebody could always speak louder than a person further away.(drowns(go down) out ) Precise power control is needed to receive all senders with the same strength at a receiver.

Access methods SDMA/FDMA/TDMA Universität Karlsruhe Institut für Telematik Access methods SDMA/FDMA/TDMA Mobilkommunikation SS 1998 SDMA (Space Division Multiple Access) is used for allocating a separated space to users in wireless networks. A typical application involves assigning an optimal base station to a mobile phone user. The mobile phone may receive several base stations with different quality. A MAC algorithm could now decide which base station is best, taking into account which frequencies (FDM), time slots (TDM) or code (CDM) are still available (depending on the technology). Typically, SDMA is never used in isolation but always in combination with one or more other schemes. basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute/set up the infrastructure implementing SDM new application of SDMA comes up together with beam-forming antenna arrays Single users are separated in space by individual beams. This can improve the overall capacity of a cell Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 15

FDMA (Frequency Division Multiple Access) comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM) scheme assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) Channels can be assigned to the same frequency at all times, i.e., pure FDMA, or change frequencies according to a certain pattern, i.e., FDMA combined with TDMA. The latter example is the common practice for many wireless systems to circumvent narrowband interference at certain frequencies, known as frequency hopping. Sender and receiver have to agree on a hopping pattern, otherwise the receiver could not tune to the right frequency. Hopping patterns are typically fixed, at least for a longer period. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.7

FDM is often used for simultaneous access to the medium by base station and mobile station in cellular networks. Here the two partners typically establish a duplex channel, i.e., a channel that allows for simultaneous transmission in both directions. The two directions, mobile station to base station and vice versa are now separated using different frequencies. This scheme is then called frequency division duplex (FDD). They should know the frequencies in advance. Uplink frequency, i.e., from mobile station to base station or from ground control to satellite, and as downlink frequency, i.e., from base station to mobile station or from satellite to ground control. In a mobile phone network based on the GSM standard for 900 MHz , basic frequency allocation scheme for GSM is fixed and regulated by national authorities. All uplinks use the band between 890.2 and 915 MHz, all downlinks use 935.2 to 960 MHz. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.8

FDD/FDMA - general scheme, example GSM UL & DL have a fixed relation. If the uplink frequency is fu = 890 MHz + n·0.2 MHz, the downlink frequency is fd = fu +45 MHz, i.e., fd = 935 MHz + n·0.2 MHz for a certain channel n. The base station selects the channel. Each channel (uplink and downlink) has a bandwidth of 200 kHz. This illustrates the use of FDM for multiple access (124 channels per direction are available at 900 MHz) Similar FDM schemes for FDD are implemented in AMPS, IS-54, IS-95, IS-136, PACS, and UMTS (FDD mode). Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.9

Universität Karlsruhe Institut für Telematik Access method : CDMA Mobilkommunikation SS 1998 CDMA (Code Division Multiple Access) is a communication technique that allows multiple users to communicate simultaneously over one frequency. This is achieved through the use of spreading codes, whereby a single data bit is "spread" over a longer sequence of transmitted bits. These codes, known as chip sequences, must be carefully chosen so that the data may be correctly "despread" at the receiver. Such codes are known as orthogonal codes. all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel each sender has a unique random number, the sender XORs the signal with this random number the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation/association function Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.10 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 28

Two vectors are called orthogonal if their inner product is 0, as is the case for the two vectors (2, 5, 0) and (0, 0, 17): (2, 5, 0)*(0, 0, 17) = 0 + 0 + 0 = 0. But also vectors like (3, –2, 4) and (–2, 3, 3) are orthogonal: (3, –2, 4)*(–2, 3, 3) = –6 – 6 + 12 = 0. By contrast, the vectors (1,2,3) and (4,2, –6) are not orthogonal (the inner product is –10), while (1, 2, 3) and (4, 2, –3) are “almost” orthogonal, with their inner product being –1 (which is “close” to zero) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.11

Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g. 232) compared to frequency space interferences (e.g. white noise) is not coded forward error correction and encryption can be easily integrated correlation is one of the most common and most useful statistics. A correlation is a single number that describes the degree of relationship between two variables Autocorrelation is also sometimes called ``lagged correlation'' or ``serial correlation'', which refers to the correlation between members of a series of numbers arranged in time. (x1,x2,x3…..xn) | (y1,y2, y3….. yn) Disadvantages: higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) all signals should have the same strength at a receiver

Prof. Dr. -Ing. Jochen Schiller, http://www. jochenschiller. de/ Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.13

Transmitted bits, data = 1: Each user is assigned a chip sequence. This is simply a sequence of bits (chips). The spreading process is shown below, using an example: Chip Sequence: 1 0 1 1 0 0 Spreading Sequence: 1 -1 1 1 -1 -1 Transmitted bits, data = 1: Transmitted bits, data = 0: -1 1 -1 -1 1 1 No transmission: 0 0 0 0 0 0 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.14

Prof. Dr. -Ing. Jochen Schiller, http://www. jochenschiller. de/ Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.15

CDMA in theory assume that we code a binary 0 as –1, a binary 1 as +1. Sender A wants to send the bit Ad = 1, key Ak = 010011 sending signal As = Ad *Ak = +1*(–1, +1, –1, –1, +1, +1) = (–1, +1, –1, –1, +1, +1). Sender B wants to send the bit Bd = 0, key Bk = 110101 sending signal Bs = Bd * Bk = –1*(+1, +1, –1, +1, –1, +1) = (–1, –1, +1, –1, +1, –1). Both signals are then transmitted at the same time using the same frequency, so the signals superimpose in space Discounting interference from other senders and environmental noise from this simple example, and assuming that the signals have the same strength at the receiver, the following signal C is received at a receiver: C = As + Bs = (–2, 0, 0, –2, +2, 0).

Receiver now wants to receive data from sender A and, therefore, tunes in to the code of A, i.e., applies A’s code for despreading: C*Ak = (–2, 0, 0, –2, +2, 0)*(–1, +1, –1, –1, +1, +1) = 2 + 0 + 0 + 2 + 2 + 0 = 6. As the result is much larger than 0, the receiver detects a binary 1. Tuning in to sender B, i.e., applying B’s code gives C*Bk = (–2, 0, 0, –2, +2, 0)*(+1, +1, –1, +1, –1, +1) = –2 + 0 + 0 – 2 – 2 + 0 = –6. The result is negative, so a 0 has been detected. Here codes were extremely simple, but at least orthogonal. More importantly, noise was neglected. Noise would add to the transmitted signal C, the results would not be as even with –6 and +6, but would maybe be close to 0, making it harder to decide Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.17

C'*Bk = (–6, –4, +4, –6, +6, –4)*(+1, +1, –1, +1, –1, +1) What would happen if, for example, B was much stronger? Assume that B’s strength is five times A’s strength. Then, C' = As + 5*Bs = (–1, +1, –1, –1, +1, +1) + (–5, –5, +5, –5, +5, –5) = (–6, –4, +4, –6, +6, –4). Again, a receiver wants to receive B: C'*Bk = (–6, –4, +4, –6, +6, –4)*(+1, +1, –1, +1, –1, +1) = –6 –4 – 4 – 6 – 6 – 4= –30. It is easy to detect the binary 0 sent by B. Now the receiver wants to receive A: C'*Ak = (–6, –4, +4, –6, +6, –4) *(–1, +1, –1, –1, +1, +1) = 6 – 4 – 4 + 6 + 6 – 4 = 6. Clearly, the (absolute) value for the much stronger signal is higher (30 compared to 6). While –30 might still be detected as 0, this is not so easy for the 6 because compared to 30, 6 is quite close to zero and could be interpreted/understood as noise. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.18

If one person speaks in one language very loudly, it is of no more use to have another language as orthogonal code – no one can understand you, your voice will only add to the noise. this example shows that power control is essential for CDMA systems. This is one of the biggest problems CDMA systems face as the power has to be adjusted over one thousand times per second in some systems – this consumes a lot of energy Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.19

CDMA on signal level I Ad 1 1 1 1 1 1 1 1 1 1 Ak 1 1 1 1 1 1 1 1 As data A Ad 1 1 key A key sequence A 1 1 1 1 1 1 1 1 Ak data  key 1 1 1 1 1 1 1 1 As signal A Real systems use much longer keys resulting in a larger distance between single code words in code space. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.20

CDMA on signal level II As Bd 1 1 1 1 1 1 1 1 1 Bk 1 1 1 1 1 1 1 1 1 1 signal A As data B Bd 1 key B key sequence B 1 1 1 1 1 1 1 1 Bk 1 1 1 1 1 1 1 1 1 1 data  key Bs signal B As + Bs Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.21

CDMA on signal level III data A Ad 1 1 As + Bs Ak (As + Bs) * Ak integrator output comparator output 1 1 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.22

CDMA on signal level IV Bd 1 1 data B As + Bs Bk (As + Bs) * Bk As + Bs Bk (As + Bs) * Bk integrator output comparator output 1 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.23

CDMA on signal level V (0) (0) ? As + Bs wrong key K (As + Bs) * K integrator output comparator output (0) (0) ? Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.24

SAMA - Spread Aloha Multiple Access Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha collision sender A 1 1 narrow band sender B 1 1 send for a shorter period with higher power spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“) t Problem: find a chipping sequence with good characteristics Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.25

TDMA (Time Division Multiple Access) Compared to FDMA, Time Division Multiple Access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication. Assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time. Now tuning in to a certain frequency is not necessary, i.e., the receiver can stay at the same frequency the whole time. As already mentioned, listening to different frequencies at the same time is quite difficult, but listening to many channels separated in time at the same frequency is simple. e.g., Ethernet, Token Ring, ATM etc. The multiplexing schemes presented in chapter 2 are now used to control medium access! Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.26

Now synchronization between sender and receiver has to be achieved in the time domain allocating a certain time slot for a channel, or by using a dynamic allocation scheme. Dynamic allocation schemes require an identification for each transmission as this is the case for typical wired MAC schemes (e.g., sender address) or the transmission has to be announced beforehand. MAC addresses are quite often used as identification. This enables a receiver in a broadcast medium to recognize if it really is the intended receiver of a message. Fixed schemes do not need an identification, but are not as flexible considering varying bandwidth requirements. Following sections present several examples for fixed and dynamic schemes as used for wireless transmission. Typically, those schemes can be combined with FDMA to achieve even greater flexibility and transmission capacity. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.27

1. Fixed TDM The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern. This results in a fixed bandwidth and is the typical solution for wireless phone systems. MAC is quite simple, as the only crucial factor is accessing the reserved time slot at the right moment. If this synchronization is assured, each mobile station knows its turn and no interference will happen. The fixed pattern can be assigned by the base station, where competition between different mobile stations that want to access the medium is solved. Fixed access patterns (at least fixed for some period in time) fit perfectly well for connections with a fixed bandwidth. Furthermore, these patterns guarantee a fixed delay – one can transmit, e.g., every 10 ms as this is the case for standard DECT systems. TDMA schemes with fixed access patterns are used for many digital mobile phone systems like IS-54, IS-136, GSM, DECT, PHS, and PACS. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.28

Following figure shows how these fixed TDM patterns are used to implement multiple access and a duplex channel between a base station and mobile station. Assigning different slots for uplink and downlink using the same frequency is called time division duplex (TDD). As shown in the figure, the base station uses one out of 12 slots for the downlink, whereas the mobile station uses one out of 12 different slots for the uplink. Uplink and downlink are separated in time. Up to 12 different mobile stations can use the same frequency without interference using this scheme. Each connection is allotted its own up- and downlink pair. In the example below, which is the standard case for the DECT cordless phone system, the pattern is repeated every 10 ms, i.e., each slot has a duration of 417 μs. This repetition guarantees access to the medium every 10 ms, independent of any other connections.

417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.30

It is too static, too inflexible for data communication. While the fixed access patterns, as shown for DECT, are perfectly apt for connections with a constant data rate (e.g., classical voice transmission with 32 or 64 kbit/s duplex), but they are very inefficient for bursty data or asymmetric connections If temporary bursts in data are sent from the base station to the mobile station often or vice versa (web page-read: ok; click-url/hy.link:data transfer-MS-BS-Web Server) If allocate asymmetric bandwidth ,this general scheme still wastes a lot of bandwidth It is too static, too inflexible for data communication. In this case, connectionless, demand-oriented TDMA schemes can be used, as the following sections show. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.31

2. Classical ALOHA TDMA comprises all mechanisms controlling medium access according to TDM. But what happens if TDM is applied without controlling access!! This is exactly what the classical Aloha scheme does- was invented at the University of Hawaii and was used in the ALOHANET for wireless connection of several stations. Aloha neither coordinates medium access nor does it resolve contention on the MAC layer. Instead, each station can access the medium at any time as shown in Figure. This is a random access scheme, without a central arbiter controlling access and without coordination among the stations. If two or more stations access the medium at the same time, a collision occurs and the transmitted data is destroyed. Resolving this problem is left to higher layers (e.g., retransmission of data).

3. Slotted ALOHA simple ALOHA works fine for a light load and does not require any complicated access mechanisms. On the classical assumption maximum throughput :18 per cent Slotted Aloha additionally uses time-slots, In this case, all senders have to be synchronized, transmission can only start at the beginning of a time slot as shown in Figure In the following sections, both basic ALOHA principles occur in many systems that implement distributed access to a medium. Aloha systems work perfectly well under a light load), but they cannot give any hard transmission guarantees, such as maximum delay before accessing the medium, or minimum throughput. Here one needs additional mechanisms,e.g., combining fixed schemes and Aloha schemes. However, even new mobile communication systems like UMTS have to rely on slotted Aloha for medium access in certain situations (random access for initial connection set-up).

4. CARRIER SENSE MULTIPLE ACCESS One improvement to the basic Aloha is sensing the carrier before accessing the medium. This is what carrier sense multiple access (CSMA) Sensing the carrier and accessing the medium only if the carrier is idle decreases the probability of a collision. But, as already mentioned in the introduction, hidden terminals cannot be detected, so, if a hidden terminal transmits at the same time as another sender, a collision might occur at the receiver. This basic scheme is still used in most wireless LANs Several versions of CSMA: non-persistent CSMA, stations sense the carrier and start sending immediately if the medium is idle. If the medium is busy, the station pauses a random amount of time before sensing the medium again and repeating this pattern Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.34

In p-persistent CSMA systems, nodes also sense the medium, but only transmit with a probability of p, with the station deferring/accepting to the next slot with the probability 1-p, i.e., access is slotted in addition In 1-persistent CSMA systems, all stations wishing to transmit - access the medium at the same time, as soon as it becomes idle. This will cause many collisions if many stations wish to send and block each other. To create some fairness for stations waiting for a longer time, back-off algorithms can be introduced, which are sensitive to waiting time as this is done for standard Ethernet. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.35

CSMA with collision avoidance (CSMA/CA) is one of the access schemes used in wireless LANs following the standard IEEE 802.11. Here sensing the carrier is combined with a back-off scheme in case of a busy medium to achieve some fairness among competing stations. Another, very elaborate scheme is elimination yield – non-preemptive multiple access (EY-NMPA) used in the HIPERLAN 1 specification. Here several phases of sensing the medium and accessing the medium for contention resolution are interleaved before one “winner” can finally access the medium for data transmission. Here, priority schemes can be included to assure preference of certain stations with more important data. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.36

DAMA - Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) Reservation can increase efficiency to 80% a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links Examples for reservation algorithms: Explicit Reservation according to Roberts (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.37

Access method DAMA: Explicit Reservation Universität Karlsruhe Institut für Telematik Access method DAMA: Explicit Reservation Mobilkommunikation SS 1998 Explicit Reservation (Reservation Aloha): two modes: ALOHA mode for reservation: competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible) it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision t Aloha reserved Aloha reserved Aloha reserved Aloha Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.38 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 24

6.PRMA Packet Reservation Multiple Access Universität Karlsruhe Institut für Telematik 6.PRMA Packet Reservation Multiple Access Mobilkommunikation SS 1998 An example for an implicit reservation scheme is PRMA A certain number of slots forms a frame (Figure shows eight slots in a frame). The frame is repeated in time (forming frames one to five in the example), i.e., a fixed TDM pattern is applied. base station, which could be a satellite, now broadcasts the status of each slot (as shown on the left side of the figure) to all mobile stations. All stations receiving this vector will then know which slot is occupied and which slot is currently free. a successful transmission of data is indicated by the station’s name (A to F). In the example, the BS broadcasts the reservation status ‘ACDABA-F’ to all stations, here A to F. This means that slots one to six and eight are occupied, but slot seven is free Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.39 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 25

Prof. Dr. -Ing. Jochen Schiller, http://www. jochenschiller. de/ Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.40

All stations wishing to transmit can now compete for this free slot in Aloha fashion. The already occupied slots are not touched. In the example shown, more than one station wants to access this slot, so a collision occurs. The base station returns the reservation status ‘ACDABA-F’, indicating that the reservation of slot seven failed (still indicated as free) and that nothing has changed for the other slots. Again, stations can compete for this slot. Additionally, station D has stopped sending in slot three and station F in slot eight. This is noticed by the base station after the second frame. the third frame starts, the base station indicates that slots three and eight are now idle. Station F has succeeded in reserving slot seven as also indicated by the base station.

This ensures transmission with a guaranteed data rate. PRMA constitutes yet another combination of fixed and random TDM schemes with reservation compared to the previous schemes. As soon as a station has succeeded with a reservation, all future slots are implicitly reserved for this station. This ensures transmission with a guaranteed data rate. The slotted aloha scheme is used for idle slots only, data transmission is not destroyed by collision Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.42

Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) This guarantees each station a certain bandwidth and a fixed delay. Other stations can now send data in unused data-slots as shown. Using these free slots can be based on a simple round-robin scheme or can be uncoordinated using an Aloha scheme. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.43

Access method DAMA: Reservation-TDMA Universität Karlsruhe Institut für Telematik Access method DAMA: Reservation-TDMA Mobilkommunikation SS 1998 e.g. N=6, k=2 N * k data-slots N mini-slots reservations for data-slots other stations can use free data-slots based on a round-robin scheme Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.44 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 26

MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.45

MACA examples MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, while C to another terminal now C does not have to wait for it ., because it cannot receive CTS from A A C RTS CTS CTS B A C RTS RTS CTS B Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.46

MACA variant: DFWMAC in IEEE802.11 sender receiver idle idle packet ready to send; RTS data; ACK RxBusy time-out; RTS wait for the right to send RTS; CTS time-out  data; NAK ACK time-out  NAK; RTS CTS; data wait for data wait for ACK RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.47

9 . Polling mechanisms Where one station is to be heard by all others (e.g., the base station of a mobile phone network or any other dedicated station), polling schemes can be applied. Polling is a strictly centralized scheme with one master station and several slave stations. The master can poll / census / Election the slaves according to many schemes: round robin (only efficient if traffic patterns are similar over all stations), randomly, according to reservations (the classroom example with polite students) etc. master could also establish a list of stations wishing to transmit during a contention phase. After this phase, the station polls each station on the list Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.48

Example: Randomly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send , can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminal this cycle starts again after polling all terminals of the list Similar schemes are used, e.g., in the Bluetooth wireless LAN and as one possible access function in IEEE 802.11 systems Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.49

10. ISMA (Inhibit Sense Multiple Access) Another combination of different schemes is represented by inhibit sense multiple access (ISMA). which is used for the packet data transmission service Cellular Digital Packet Data (CDPD) in the AMPS mobile phone system Also known as digital sense multiple access (DSMA). base station only signals a busy medium via a busy tone (called BUSY/IDLE indicator) on the downlink Current state of the medium is signaled via a “busy tone” terminals must not send, if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals “collisions and successful transmissions” via the busy tone accessing the uplink is not coordinated any further. base station acknowledges successful transmissions, a mobile station detects a collision only via the missing positive acknowledgement. In case of collisions, additional back-off and retransmission mechanisms are implemented.

Prof. Dr. -Ing. Jochen Schiller, http://www. jochenschiller. de/ Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.51

Comparison SDMA/TDMA/FDMA/CDMA Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.52